1. Field of the Invention
This invention relates to a differential interference microscope which may be used, for example, to observe microscopic bumps on a metal surface and/or phase objects within biological cells.
2. Description of Related Art
Conventionally, differential interference microscopes have been used to observe micro phase objects. An example of such conventional microscopes is shown in FIG. 8, which is a vertical illumination type differential interference microscope.
As shown in FIG. 8, a ray emitted from a light source 10 passes through a condenser lens 11 and a polarizer 12, which produces a linearly polarized ray 1. The linearly polarized ray 1 is reflected by a half-mirror 14 toward the object surface 17. The ray 1 forms a light-source image near the back focal point of the object lens 16. The ray 1 further passes through a Wollaston prism 15, which is positioned near the light-source image, and the objective lens 16, and illuminates the object surface 17 from above. In this case, the Wollaston prism 15 splits the ray 1 into an ordinary ray 2 and an extraordinary ray 3 having electric field oscillation planes which are perpendicular to each other. The ordinary ray 2 and the extraordinary ray 3 spread out with a certain angle therebetween. In FIG. 8, the oscillation plane of the electric field of the ordinary ray 2 is parallel to the paper plane, while the oscillation plane of the electric field of the extraordinary ray 3 is perpendicular to the paper plane.
If the polarization directions of the ordinary ray 2 and the extraordinary ray 3 are 45 degree-angles with respect to the optic axis of the polarizer 12, then the light quantities of the ordinary ray 2 and the extraordinary ray 3 become substantially equal. In addition, the Wollaston prism 15 is positioned in such a way that the center point 15a from which the ordinary ray 2 and extraordinary ray 3 diverge is in agreement with the back focal point of the objective lens 16. In this arrangement, the ordinary ray component 2 and the extraordinary ray component 3 of the light emitted from a point of the light source 10 pass through the objective lens 16 and telecentrically illuminate spots which are slightly apart from each other on the object surface 17.
The ordinary ray 2 and the extraordinary ray 3 that have illuminated the object surface 17 from above are reflected by the object surface 17. The reflected rays are depicted as an ordinary ray 4 and an extraordinary ray 5, respectively, in FIG. 8. The ordinary ray 4 and the extraordinary ray 5 pass through the objective lens 16 again, and they cross again at the center 15a at which the initial ray was split into the ordinary ray 2 and the extraordinary ray 3. That is, the center point 15a, to which the reflected ordinary ray 4 and the extraordinary ray 5 converge, and from which the ordinary ray 2 and the extraordinary ray 3 diverged, is self-conjugate with respect to the objective lens 16 and the object surface 17. The converged ordinary ray 4 and the extraordinary ray 4 pass through the Wollaston prism 15, and are composed into a composite ray 6. The ray 6 passes through the half-mirror 14 and the analyzer 18. In this process, only a linearly polarized component that is parallel to the optic axis of the analyzer 18 passes through the analyzer. This component further passes through a focusing lens 19 to form an image on an image plane 20. As has been described, the ordinary ray 2 and the extraordinary ray 3 strike the object surface 17 at positions slightly apart from each other, and the ordinary ray 4 and the extraordinary ray 5 reflected by these points form an image on the image plane 20. Accordingly, if there are bumps or unevenness on the object surface 17, interference fringes are formed on the image plane 20 because the optical path-lengths of the ordinary ray 4 and the extraordinary ray 5 between the object surface 17 to the image plane 17 become different, which causes a phase difference.
In this case, if the direction of the optic axis of the analyzer 18 is set parallel to that of the polarizer 12, then the image becomes brightest when the phase difference between the ordinary ray 4 and the extraordinary ray 5 is 2 .pi.n (n=0, 1, 2, . . . ). If the optic axis of the analyzer 18 is made orthogonal to the optic axis of the polarizer 12, then the image becomes brightest when the phase difference between the ordinary ray 4 and the extraordinary ray 5 is n+2 .pi.n (n=0, 1, 2, . . . ). Hence, we can know the depth or the height of the bump located at an arbitrary point on the object plane 17 by observing a contrast pattern formed on the image plane 20.
In order to observe the bump on the object surface 17 in more detail, the wavelength of the ray is changed. The phase difference changes as the wavelength is varied, which results in a change in the contrast pattern. The bump on the object surface 17 can be detected with high accuracy by observing the change in the contrast pattern. Alternatively, the bump on the object surface 17 can be detected by using a white light and observing bright colors.
The conventional differential interference microscope mentioned above requires visual observation of interference patterns. This prevents one from achieving a degree of measurement accuracy beyond the magnitude of the wave length of the ray used for the observation. Such a degree of measurement accuracy is not satisfactory. Moreover, it is difficult for the conventional differential interference microscope to treat interference patterns quantitatively. Another problem in the conventional differential interference microscope is that the use of a light source having a broad range of wavelengths reduces the spatial resolution of the microscope due to the chromatic aberrations of the lenses. In addition, any reflectance variations on the object surface diminishes the SN ratio of the image, which makes observation of the interference patterns difficult.